This invention relates in general to a new and improved process for making extruded thermoplastic members reinforced with an embedded helically formed wire, such as, particularly, with respect to pipes, and to a device for effecing such process.
Heretofore, various expedients have been developed in an attempt to reinforce thermoplastic members with metal wires or with synthetic fibers, but the same have consistently proved inadequate. One such effort involved the utilization of a pair of spaced apart extrusion heads for the thermoplastic material whereby an inner pipe section was formed by the first extrusion head and the reinforcing member, such as wire, was wound about the exterior thereof as the same was being advanced to the second extrusion head. In the latter head an outer pipe section or second thermoplastic layer was applied upon the wire wound inner section. This equipment had the obvious drawback of requiring a pair of extrusion heads with their respective feeding devices and with an independent device therebetween for applying the wire. Furthermore pipes produced by such equipment were potentially weak in the zone of the interface between the inner and outer pipe sections as the bond therebetween was of but limited resistance to mechanical stresses. For example, with the development of a high vacuum within the pipe, the two sections would part, thus, negating the expected reinforcing effect of the wire with the pipe becoming useless.
Another machine which is in current use incorporating a single extrusion head causes the helical wire to be fed to the head in such manner as to effect jamming in the injection chamber due to the resistance of the thermoplastic material whereby the wire is compressed and the turns or coils are displaced from the longitudinal axis of the spiral. In attempts to minimize such jamming potential the thermoplastic material is injected in a rather highly liquefied state markedly remote from the optimal densities and temperatures normally preferred. Also, such equipment is devoid of a sizing run for the finished pipe and merely includes a relatively short run as in the order of 2 to 4 mm at the outlet of the extrusion head. Such constructional character was contrived to reduce the compression of the thermoplastic material which might tend to force the wire coils from the desired uniform diameter.
Such currently known devices, including the foregoing, do not produce a pipe having a wall of constant cross-section or thickness and with inner and outer surface regularity since the pipes produced have walls with an undulating or wavy character with the surfaces relatively rough and highly irregular. Pipes with such manifest defects are productive of losses of charge and entail union sealing problems requiring, as for example, locking by means of compression rings and the like. Another disadvantage is that pipes produced by existing equipment must perforce involve walls of relative thickness to counteract any axial displacement of the wire coils so as to prevent portions thereof from projecting through the outer surface of the pipe.
Therefore, it is an object of the present invention to provide a process for producing extruded thermoplastic bodies having a reinforcing means provided therein such as helically formed wire which are devoid of the deficiencies present in like members produced in accordance with existing equipment or currently practiced processes.
It is another object of the present invention is to provide a device for the purpose stated which incorporates but a single extrusion head and with means for presenting the reinforcing agents, such as, a coil wire, to the thermoplastic so as to assure of consistent centering of the same within the formed body.
It is another object of the present invention to provide a device for the purpose stated which is adapted for producing extruded pipe of thermoplastic material having sized, smooth surfaced, relatively thin walls which are of constant cross-section and wherein the embedded reinforcing member, such as a coil wire, is substantially equidistant from the inner and outer wall surfaces.
It is a further object of the present invention to provide a method for embedding the reinforcing member within a thermoplastic body which may be economically practiced and readily performed by relatively unskilled individuals and one wherein requisite positioning of the reinforcing member is consistently achieved.
It is another object of the present invention to provide a device of the character stated which is so constructed as to be resistant to malfunctioning, jamming, or the like by reason of the unique relationship of the components.
It is another object of the present invention to provide a device of the character stated which is comprised of a simplicity of parts; which is economical in operation; and which is durable and reliable in use.